Modeling AeroForm tissue expander for postmastectomy radiation therapy.

The AeroForm chest wall tissue expander (TE) is a silicon shell containing a metallic CO2 reservoir, placed surgically after mastectomy. The patient uses a remote control to release compressed CO2 from the reservoir to inflate the expander. AeroForm poses challenges in a radiation therapy setting: The high density of the metallic reservoir causes imaging artifacts on the planning CT, which encumber structure definition and cause misrepresentation of density information, in turn affecting dose calculation. Additionally, convolution-based dose calculation algorithms may not be well-suited to calculate dose in and around high-density materials. In this study, a model of the AeroForm TE was created in Eclipse treatment planning system (TPS). The TPS model was validated by comparing measured to calculated transmission through the AeroForm. Transmission was measured with various geometries using radiochromic film. Dose was calculated with both Varian's Anisotropic Analytical Algorithm (AAA) and Acuros External Beam (AXB) algorithms. AAA and AXB were compared using dose profile and gamma analyses. While both algorithms modeled direct transmission well, AXB better modeled lateral scatter from the AeroForm TE. Clinical significance was evaluated using clinical data from four patients with AeroForm TEs. The AeroForm TPS model was applied, and RT plans were optimized using AAA, then re-calculated with AXB. Structures of clinical significance were defined and dose volume histogram analysis was performed. Compared to AXB, AAA overestimates dose in the AeroForm device. Changes in clinically significant regions were patient- and plan-specific. This study proposes a clinical procedure for modeling the AeroForm in a commercial TPS, and discusses the limitations of dose calculation in and around the device. An understanding of dose calculation accuracy in the vicinity of the AeroForm is critical for assessing individual plan quality, appropriateness of different planning techniques and dose calculation algorithms, and even the decision to use the AeroForm in a postmastectomy radiation therapy setting.

Dziemianowicz E ，Gardner SJ ，Chin Snyder K ，Wen N ，Walker EM ，Fraser C ，Reding A ，Chetty IJ ... -  《Journal of Applied Clinical Medical Physics》

A dosimetry study of post-mastectomy radiation therapy with AeroForm tissue expander.

To evaluate the dosimetric effects of the AeroFormTM (AirXanpders®, Palo Alto, CA) tissue expander in-situ for breast cancer patients receiving post-mastectomy radiation therapy. A film phantom (P1) was constructed by placing the metallic canister of the AeroForm on a solid water phantom with EBT3 films at five depths ranging from 2.6 mm to 66.2 mm. A breast phantom (P2), a three-dimensional printed tissue-equivalent breast with fully expanded AeroForm in-situ, was placed on a thorax phantom. A total of 21 optical luminescent dosimeters (OLSDs) were placed on the anterior skin-gas interface and the posterior chest wall-metal interface of the AeroForm. Both phantoms were imaged with a 16-bit computed tomography scanner with orthopedic metal artifact reduction. P1 was irradiated with an open field utilizing 6 MV and 15 MV photon beams at 0°, 90°, and 270°. P2 was irradiated using a volumetric modulated arc therapy plan with a 6 MV photon beam and a tangential plan with a 15 MV photon beam. All doses were calculated using Eclipse (Varian, Palo Alto, CA) with AAA and AcurosXB (AXB) algorithms. The average dose differences between film measurements and AXB in the region adjacent to the canister in P1 were within 3.1% for 15 MV and 0.9% for 6 MV. Local dose differences over 10% were also observed. In the chest wall region of P2, the median dose of OLSDs in percentage of prescription dose were 108.4% (range 95.4%-113.0%) for the 15MV tangential plan and 110.4% (range 99.1%-113.8%) for the 6MV volumetric modulated arc therapy plan. In the skin-gas interface, the median dose of the OLSDs were 102.3% (range 92.7%-107.7%) for the 15 MV plan and 108.2% (range 97.8-113.5%) for the 6 MV plan. Measured doses were, in general, higher than calculated doses with AXB calculations. The AAA dose algorithms produced results with slightly larger discrepancies between measurements compared with AXB. The AeroForm creates significant dose uncertainties in the chest wall-metal interface. The AcurosXB dose calculation algorithm is recommended for more accurate calculations. If possible, post-mastectomy radiation therapy should be delivered after the permanent implant is in place.

Lim SB ，Kuo LC ，Li G ，Kuo HC ，McCormick B ，Cahlon O ，Powell S ，Hong LX ... -  《Journal of Applied Clinical Medical Physics》

Dosimetric impact of the AeroForm tissue expander in postmastectomy radiation therapy: an ex vivo analysis.

To evaluate the effect of the AeroForm (AirXpanders Inc, Palo Alto, CA) tissue expander on the dose distribution in a phantom from a simulated postmastectomy radiation treatment for breast cancer. Experiments were conducted to determine the effect on the dose distribution with the metallic reservoir irradiated independently and with the entire AeroForm tissue expander placed on a RANDO phantom (The Phantom Laboratory, Salem, NY). The metallic reservoir was irradiated on a block of solid water with film at various depths ranging from 0 to 8.2 cm from the surface. The intact 400 cc AeroForm was inflated to full capacity and irradiated while positioned on a RANDO phantom, with 12 optically stimulated luminescent dosimeters (OSLDs) placed at clinically relevant expander-tissue interface points. Film dosimetry with the reservoir perpendicular to film reveals 40% transmission at a depth of 0.7 cm, which increases to 60% at a depth of 8.2 cm. In the parallel position, the results vary depending on which area under the reservoir is examined, indicating that the reservoir is not a uniformly dense object. Testing of the intact expander on the phantom revealed that the average percent difference (measured vs expected dose) was 2.7%, σ = 6.2% with heterogeneity correction and 3.7%, σ = 2.4% without heterogeneity correction. The only position where the OSLD readings were consistently higher than the calculated dose by >5% was at position 1, just deep to the canister at the expander-phantom interface. At this position, the readings varied from 5.2% to 14.5%, regardless of heterogeneity correction. Film dosimetry demonstrated beam attenuation in the shadow of the metallic reservoir in the expander. This decrease in dose was not reproduced on the intact expander on the phantom designed to replicate a clinical setup.

Moni J ，Saleeby J ，Bannon E ，Lo YC ，Fitzgerald TJ ... -  《-》

Dosimetric comparison of analytical anisotropic algorithm and the two dose reporting modes of Acuros XB dose calculation algorithm in volumetric modulated arc therapy of carcinoma lung and carcinoma prostate.

Volumetric Modulated Arc Therapy (VMAT) is an important modality for radical radiotherapy of all major treatment sites. This study aims to compare Analytical Anisotropic Algorithm (AAA) and the two dose-reporting modes of Acuros XB (AXB) algorithm -the dose to medium option (Dm) and the dose to water option (Dw) in Volumetric Modulated Arc Therapy (VMAT) of carcinoma lung and carcinoma prostate. We also compared the measured dose with Treatment Planning System calculated dose for AAA and the two dose reporting options of Acuros XB using Electronic Portal Imaging Device (EPID) and ArcCHECK phantom. Treatment plans of twenty patients each who have already undergone radiotherapy for cancer of lung and cancer of prostate were selected for the study. Three sets of VMAT plans were generated in Eclipse Treatment Planning System (TPS), one with AAA and two plans with Acuros-Dm and Acuros-Dw options. The Dose Volume Histograms (DVHs) were compared and analyzed for Planning Target Volume (PTV) and critical structures for all the plans. Verification plans were created for each plan and measured doses were compared with TPS calculated doses using EPID and ArcCHECK phantom for all the three algorithms. For lung plans, the mean dose to PTV in the AXB-Dw plans was higher by 1.7% and in the AXB-Dm plans by 0.66% when compared to AAA plans. For prostate plans, the mean dose to PTV in the AXB-Dw plans was higher by 3.0% and in the AXB-Dm plans by 1.6% when compared to AAA plans. There was no difference in the Conformity Index (CI) between AAA and AXB-Dm and between AAA and AXB-Dw plans for both sites. But the homogeneity worsened in AXB-Dw and AXB-Dm plans when compared to AAA plans for both sites. AXB-Dw calculated higher dose values for PTV and all the critical structures with significant differences with one or two exceptions. Point dose measurements in ArcCHECK phantom showed that AXB-Dm and AXB-Dw options showed very small deviations with measured dose distributions than AAA for both sites. Results of EPID QA also showed better pass rates for AXB-Dw and AXB-Dm than AAA for both sites when gamma analysis was done for 3%/3 mm and 2%/2 mm criteria. With reference to the results, it is always better to choose Acuros algorithm for dose calculations if it is available in the TPS. AXB-Dw plans showed very high dose values in the PTV when compared to AAA and AXB-Dm in both sites studied. Also, the volume of PTV receiving 107% dose was significantly high in AXB-Dw plans compared to AXB-Dm plans in sites involving high density bones. Considering the results of dosimetric comparison and QA measurements, it is always better to choose AXB-Dm algorithm for dose calculations for all treatment sites especially when high density bony structures and complex treatment techniques are involved. For patient specific QA purposes, choosing AXB-Dm or AXB-Dw does not make any significant difference between calculated and measured dose distributions.

Gopalakrishnan Z ，Bhasi S ，P R ，Menon SV ，B S ，Thayil AG ，Nair RK ... -  《-》

Technical Note: Clinical modeling and validation of breast tissue expander metallic ports in a commercial treatment planning system for proton therapy.

To validate breast tissue expander metallic port (MP) models in a commercial treatment planning system (TPS) in proton pencil beam scanning (PBS) treatments for breast cancer patients with breast tissue expanders. Three types of MPs taken out of a Mentor CPX4, a Natrelle 133, and a PMT Integra breast tissue expanders and a 650 cc saline filled Mentor CPX4 expander were placed on top of acrylic slabs, and scanned using a Siemens Somatom Definition AS Open RT CT scanner. Structure templates for each of the MPs were designed within Eclipse TPS. The CT numbers for the metallic parts were overridden to reflect measured or calculated relative proton stopping powers (RPSPs). Mock targets were contoured in acrylic to represent postmastectomy chest-wall radiation therapy (PMRT) targets. Plans with different beam incident angles were optimized using the Eclipse TPS to deliver uniform prescription dose to the target using Hitachi Probeat-V PBS beams. Eclipse calculated doses and an in-house Monte Carlo (MC) code calculated doses were compared to the measured Gafchromic EBT3 film doses in acrylic. TPS/MC and film dose comparison results showed that (1) 3%/2 mm/10% threshold Gamma pass rates were better than 90.8% in the acrylic target region for all plans; (2) comparing TPS and film doses for the individual beam plans in the MP dose shadow areas, the area with dose difference above 5% ([ΔA] 5%) ranged from 1.1 to 5.0 cm2 , and the maximum dose difference ([ΔD] 0.01 cm2 ) ranged from 12.5% to 25.0%; (3) comparing MC and film doses for the individual beam plans in the MP dose shadow areas, the (ΔA) 5% varied from 1.1 to 2.9 cm2 and (ΔD) 0.01 cm2 varied from 8.5% to 24.2%; (4) for a plan composed of three individual beams treating through the Mentor CPX4 expander, the TPS (ΔA) 5% was less than 0.13 cm2 , and the (ΔD) 0.01 cm2 was less than 6% in the MP dose shadow areas. It is feasible to treat patients with tissue expanders using multiple PBS beams using a structure template with CT number overridden to represent the measured/calculated RPSP for MPs for PBS treatment planning. MC dose was more accurate than analytical dose in the areas with high dose gradient caused by the density heterogeneity of the breast tissue expander MPs.

Kang Y ，Shen J ，Bues M ，Hu Y ，Liu W ，Ding X ... -  《-》